Transmission of information by the use of light over optical fibers has found widespread use in long-haul telecommunication systems. In these one-to-one, second generation systems, optical signals are generated, transported along optical fibers and detected to regenerate the original electronic signal with as little change as possible. Fibers are substituted for coaxial or radio transmission media, all signal processing is done electronically and aside from lower cost and higher quality digital transmission, little else is obtained from the optical fiber media. Third generation optical systems will use optical fiber amplifiers, optical fiber multiplex/demultiplexers, and optical fiber splitters, couplers, filters, equalizers, switches and other optical signal processors and will exploit much more of the enormous bandwidth capacity of single-mode optical fibers.
Future use of optical fibers for wideband local distribution, metropolitan and local area networks as well as multiple access computer networks will require third generation optical fiber systems especially if digital high definition video or high bit rate data signals are involved. Designs for these future systems will go beyond simple, single-channel, point-to-point optical fiber links and will include one-to-many distribution and one-to-any multiple-access networks. These applications (1) will require significant optical signal processing without conversion to electronic signals, (2) will require use of many different wavelengths, and (3) will use a significant portion of the single-mode optical fiber bandwidth capacity of approximately 25,000 GHz which corresponds to wavelengths in the range 1.45 to 1.65 .mu.m and another 25,000 GHz which corresponds to wavelengths in the range 1.2 to 1.35 .mu.m.
An economical optical filter either fixed or tunable that is compatible with single-mode optical fibers and having bandwidths between 1 and 130 GHz with low insertion loss will be an important component in third generation systems that use wavelength division demultiplexing, wideband channel switching, Erbium-doped fiber receivers, Erbium-doped fiber lasers, or other optical processing functions. The fiber Fabry-Perot (FFP) interferometric filter is such a component.
Historically, the Fabry-Perot (FP) interferometer has provided seminal information in many scientific fields including atomic physics, material science, astronomy, lasers and optical communication. This device, first described by C. Fabry and A. Perot in 1897 (Ann. Chem. phys., 12:459-501) consists of an optical cavity between two highly reflecting, low loss, partially transmitting mirrors. Lenses are typically used to generate collimated optical beams so that divergent optical beams can be processed through the FP interferometer. Single-mode optical fibers can also be used with traditional lensed FPs except that lenses with large beam expansion ratios are required with single-mode fibers resulting in reduced stability and poor optical performance.
The FFP interferometric filter consists of two highly reflective preferably plane-parallel mirrors as in conventional FPs except a length of single-mode optical fiber extends between the mirrors. The fiber inside the cavity provides guidance, eliminates the need for collimating and focusing lenses and therefore improves stability and optical performance. Single-mode fiber pigtails make the device compatible with single-mode optical fibers and other fiber devices such as splitters, couplers and amplifiers. Early FFP interferometric filters had long cavities that made them unsuitable for most telecommunication applications.
In 1987, J. Stone and L. W. Stulz described three configurations of FFP interferometric filters (Elect. Lett., 23(15):781-783, 1987), Types I, II and III, that span a wide spectrum of bandwidths and tuning ranges.
The Type I FFP is a long cavity (1-25 cm) FFP filter in which mirrors are positioned at the ends of a continuous fiber and the fiber is stretched by piezoelectric transducers (PZTs) to produce tuning of the bandwidth (BW) over the free spectral range (FSR). These long cavity devices, while not necessarily important for applications in telecommunication systems, are of interest for sensory applications.
The Type II, short cavity FP filter is a gap resonator which has no fiber inside the optical cavity and as a consequence can exhibit significant losses. The useful cavity length limit of this filter is less than about 5 .mu.m. For this reason and since the FSR and BW are wide, the type II FFP is not well-suited for telecommunication applications.
The type III FFP has an internal waveguide of length intermediate between Type I and II FFPs (5 .mu.m to 1 cm) interposed between external fiber ends. Mirrors are positioned at an external fiber end and at one end of the waveguide. The optical cavity contains a gap the width of which can be changed to tune the filter.
Types II and III FFPs are the subject of U.S. Pat. No. 4,861,136. This patent relates to an FFP which is tuned by use of piezoelectric transducers (PZTs) to change the cavity length. In order to use PZTs to change the cavity length without changing the alignment between the mirrors of the FFP, elaborate and therefore expensive alignment brackets and fixtures have been necessary. Such tunable FFPs require up to four PZTs, which are expensive device elements, to generate a rigid balanced geometry. An advantage of PZT-actuated tuning is a relatively rapidly tuned FFP filter, which is typically capable of being tuned in 1 msec.